FIG. 14 shows basic constitution of a variable output type electric power supply apparatus 50 comprising a multi-resonant type half bridge converter (an LLC converter). In addition, FIG. 15 shows waveforms such as signals in each part of the electric power supply apparatus 50. As shown in FIG. 14, in the electric power-supply apparatus 50, a multi-resonant type half bridge converter (a switching circuit), which is constituted by two switch elements Q51 and Q52 and so on, is connected to a primary winding Np50 side of a transformer 51. In addition, diodes D53 and D54 for rectification are respectively connected to secondary windings Ns51, Ns52 sides of the transformer 51.
As shown in FIG. 15, driving signals Vg51 and Vg52 are alternately inputted to gates of the switch elements Q51 and Q52 with intervening a dead-off time (a time period during which both of the switch elements Q51 and Q52 are turned off), so that an alternating current flows in the primary winding Np50. Hereupon, a voltage across a condenser Ci is assumed as VCi. Since a resonant circuit, which is constituted by an inductor Lr and capacitors Cv and Ci, is connected to the primary winding Np50 of the transformer 51, an electric current which flows to the primary winding Np50 through the switch elements Q51 and Q52 has a waveform similar to those of IQ51 and IQ52. Attending to the waveforms of such electric currents IQ51 and IQ52, values of the electric currents are rarely varied during the time period ΔT, so that it can be regarded as a direct electric current. Therefore, electric currents rarely flow to the secondary windings Ns51 and Ns52 of the transformer 51 during the time period ΔT, so that electric currents flowing to diodes D53 and D54 have intermittent waveforms like ID 53 and ID 54. The time period ΔT is called “electric power non-transmission time period”.
Hereupon, a voltage of a direct current electric power supply 52, that is, an input voltage VIN and a voltage applied to a load 53 from the secondary windings Ns1 and Ns2 of the transformer 51 through the diodes D53 and D54, that is, an output voltage \Tout are expressed as follows. Besides, “n” designates a turn ratio (step down) of the transformer 51, “Lp” designates an excitation inductance of the transformer 51, “Lr” designates a leakage inductance of the transformer 51, “C” designates a capacity of a capacitor for resonance which is connected in a series connection, and “ΔT” designates the electric power non-transmission time period.
                              V          OUT                =                                            V              IN                        n                    ·                      1                          2              -                                                                    Δ                    ·                    π                                                        2                    ·                    Lp                                                  ⁢                                                      C                    Lr                                                                                                          [                  FORMULA          ⁢                                          ⁢          1                ]            
Hereupon, when the electric power non-transmission time period ΔT is lengthened, the output voltage VOUT rises, and when the electric power non-transmission time period ΔT is shortened, the output voltage VOUT lowers. Therefore, it is possible to vary the output voltage from this electric power supply apparatus 50 by controlling the electric power non-transmission time period ΔT. In order to control the electric power non-transmission time period ΔT, timings for switching on/off of the driving signals Vg51 and Vg52 applied to the gates of the switch elements Q51 and Q52, that is, a frequency (or cycle) of pulse signals should be varied.
This electric power supply apparatus 50 is used as a switching electric power supply of various electric apparatuses because it can control the output voltage in high efficiency while being a relatively simple circuitry. However, in order to control the output voltage extensively, it is necessary to vary the frequency of the above pulse signals largely, but there is a controllable limit ΔT=0 for the electric power non-transmission time period. In addition, when the dead-off time is shortened, the switch elements Q51 and Q52 become very likely to be switched on simultaneously. Consequently, the electric current flowing to the load 53 does not become zero, and thus, a condition of zero-voltage switch collapses and soft-switching may not be realized.
FIG. 16 a constitution of an electric power supply apparatus 50′ comprising a multi-resonant type half bridge converter to control the output voltage in a wider range, which is suggested in a prior art document 1, for example. FIG. 17 shows waveforms such as signals in each part of the electric power supply apparatus 50′. As shown in FIG. 16, switch elements Q53 and Q54 are connected in series with diodes D53 and D54 in a secondary side of a transformer 51. This electric power supply apparatus 50′ performs phase control by the switch elements Q53 and Q54 connected to the secondary side of the transformer 51. In FIG. 17, Vg51-Vg54 respectively show waveforms and timings of driving signals inputted into gates of the switch elements Q51-Q54. In addition, VQ51-VQ54 respectively show voltages of the switch elements Q51-Q54. As for the switch elements Q51-Q54, switch elements such as MOSFETs are assumed, for example, and the MOSFET has a parasitism diode and a parasitism capacity. Besides, the driving signals Vg51-Vg54 respectively correspond to driving signals Vg1, Vg2, Vg3a and Vg4a which will be mentioned later.
In this electric power supply apparatus 50′, it is assumed that a driving frequency of a switching circuit in a primary side of the transformer 51 is constant, and timings for on/off of the switch elements Q53 and Q54 are delayed to timings for on/off of the switch elements Q51 and Q52 in the primary side. Since a duty ratio of voltage waveforms generated in the secondary side of the transformer varies responding to a quantity of the delay of the timings, output control can be performed extensively.
In addition, since the driving frequency of the switching circuit in the primary side of the transformer 51 of the electric power supply apparatus 50′ shown in FIG. 16 is constant, electric power loss due to the switching circuit rarely increases in comparison with the electric power supply apparatus 50 shown in FIG. 14. However, in the electric power supply apparatus 50′, electric power losses due to the switch elements Q53 and Q54 connected to the diodes D53 and D54 for rectification in the secondary side of the transformer 51 increase. In particular, the electric power losses due to the switch elements Q53 and Q54 are the same level as the electric power losses due to the diodes D53 and D54 for rectification, so that the electric power losses in the secondary side become double, simply.